A robot system comprises an industrial robot comprising a manipulator and a control system. The manipulator comprises a robot arm with a wrist housing comprising a turn disc which is rotatably arranged relative to the wrist housing. The turn disc comprises a toolholder which is adapted for attachment of, for example, a tool for joining together at least two sheets by the influence of force and at one point. Examples of such methods of joining are spot welding, riveting and clinching. The control system comprises at least one program module and a plurality of processors which are adapted to attend to the movements of the manipulator based on instructions in a computer program. The turn disc may also be adapted for attachment of a fixture intended to hold at least two sheets to be joined together.
An industrial robot is composed of interconnected robot parts such as, for example, a stand, robot arms, an arm housing, a wrist housing, and a turn disc. Two adjoining robot parts are connected to each other so as to be rotatable in relation to each other about an axis of rotation, or are linearly displaceable in relation to each other.
During spot welding of at least two sheets with the aid of an industrial robot, it is known to use a so-called welding gun as a tool. The welding gun has a first movable electrode moving towards and away from a second fixed electrode. Common fields of use for a spot welding machine are cars, white goods, cupboards and cabinets.
There are mainly two types of welding guns: so-called X-guns and C-guns. A C-gun has the approximate shape of a C comprising two branches. A first welding electrode is movably arranged in a guide at one branch of the C and moves towards and away from an opposite second welding electrode at the opposite branch. An X-gun comprises a first movable electrode arm and an opposite fixed electrode arm. One example of an X-gun is shown in FIG. 2 and described under the heading “Description of the preferred embodiments”.
A welding gun is usually controlled by compressed aid or by a servo motor. For a welding gun controlled by compressed air, the movement of the first electrode towards and away from the second electrode is achieved with a compressed-air cylinder. Patent publication WO 01/89755 describes a welding gun controlled by a servo motor and how a movable welding electrode is brought into the welding position under the influence of the servo motor.
When joining together at least two sheets by the influence of force and at a definite point, a tool such as, for example, a welding gun, comprising a first movable electrode arm and a second fixed electrode arm, is used. The electrode arms are arranged on opposite sides of the at least two sheets that are to be joined together. For the joining to provide a joint that is centred in the thickness direction of the sheets, the free end of the second fixed arm, which comprises the electrode, is to lie in the same plane as the sheets. Usually, the position of the second fixed arm deviates from the plane of the sheets, which results in the welding causing a deformation of the sheets. This may lead to deteriorated surface quality on the finished product. The reasons for the electrode arms not being centred are, for example, that the sheet is not accurately positioned in the fixture that holds the sheet in place, or that the tip of the welding electrodes has been worn down during the course of the welding.
The corresponding problems arise also for other types of joining methods, such as, for example, during riveting and clinching, where the tool comprises at least two arms which are to join said sheets together, at least partly by the influence of force.
It is known to solve the above-mentioned problems by using a balancing system, also referred to as an equalizing system, which ensures that the second fixed arm is brought to a level that lies in the same plane as the sheets. During the balancing, a balancing system, also known as equalizer, is arranged between the tool and the turning plate of the robot. The equalizing system is, in principle, a disengagable clutch, which during the movement of the tool is in a fixed position and with the second fixed arm at a defined distance from the sheet. At the end phase of the closing movement of the arms and during the joining process, the clutch is disengaged such that the tool is able to move relative to the turning plate.
It is known to use equalizing systems that are controlled by compressed air or are electrically controlled. During compressed-air equalizing, compressed air must be supplied to the tool. A considerable volume of air is required, which results in increased operating costs. As far as electrically controlled equalizing is concerned, cabling must instead be pulled to the tool. Both types of systems give an increased installation cost and increased maintenance costs compared with the case where no equalizing system is needed. An additional disadvantage of the above-mentioned equalizing systems is that they behave differently depending on how the tool is oriented in space. This is due to the described equalizing systems being dependent on the force of gravity.
There is therefore a need to be able to equalize the arms in a tool for joining together at least two sheets in a joint without having to use compressed-air-controlled or electrically controlled equalizing systems. Further, there is a need of a process for joining that is simple and where the result of the joining process is a joint which is centred in the sheets and which hence imparts a good surface quality to the finished product.